Expert: Marc Ebelini - 11/21/2007

Question
QUESTION: My push to talk switch has fell apart it has 4 wires red black white and ground and two switches where do the wires go.Do you have a schematics?

ANSWER: You don't mention if the switch is hard wired into the system or uses a plug to go into the mike jack but I will attempt to explain both scenarios. On that PJ plug that would go into the panel is wired as follows. The tip is called the key. When the the button is pushed the tip is grounded to key the transmitter. The next portion is called the ring and it completes the circuit by connecting the mike to ground. A double pole switch is involved because the tip and ring must be separate when the switch is open.
I am reluctant to comment on the colors because some installers don't follow standard colors. But normally red is the key and white the mike. Black is electrical ground and the other could be the shield that is also grounded. They may have used two switches rather than a double pole switch to keep the key and mike separate. To verify the wires you could look at the jack in the panel and use an ohmmeter to see which wire goes the the tip and which goes to the ring. You may find the wires all white in some cases but this will help you under stand the circuit. If does not help I will see if I can dig up a typical wiring circuit. Let me know so I'll start digging. But I think my explanation explains the need for two switches in place of the double pole normally used. Remember the 1/4 jack is phone and 3/16" is the transmit.

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QUESTION: It is wired into the systems and has two jacks where you plug in headset so you can't use a single pole switch and make it work by putting red on one and white on other and tie black and ground together. Thanks in advance for your help. Ken

ANSWER: I said you need a double pole switch instead of two switches. You tie the grounds to the common and put the red on one and the white on the other. Try this. Use your ohmmeter to verify that the black and other lead goes to ground. Once you have done this, with the power on touch the red to ground this should key the transmitter. If not try the white to ground if red does not key the transmitter.  One should do the keying leaving the other one the be the mike circuit. Remember don't use an active channel when testing. What I just described you to do should answer your question. Colored wires are useless unless you have the color code. I supplied all my avionics installations with my proprietary code that was 27 different colors. If the above doesn't help maybe I can hand draw you a schematic but in any case you will need a good ohmmeter Since it's only a flow circuit. An accurate schematic can only be supplied by the person that originally wired it and selected the colors. If you will be doing any electrical work on your aircraft a good analog VOM is the most useful tool. Should still need help let me know. What sometimes appears obvious to me is not clear to others. Must remember this is all clear to me in my head because I have done it for years. Still searching hard drive for an old intercom manual that would surely help if I can find it.  


---------- FOLLOW-UP ----------

QUESTION: I tied the black and ground to one side and red and white to other and it works thanks for all your help Ken

Answer
Glad I was able to help. Don't know what was accomplished in your aircraft but PTT is very simple. What I did was to install a N/O miniature push button switch in the control wheel and ran the wires (2) through the control column and came out at a location that did not interfere with the control forward and aft positions. One wire grounded to a good source of ground and the other would go to the lug on the mike jack that picks up the tip of the mike plug. Now just plug your boom mike/headset into the jacks and your ready to PTT. This must be done by a licensed technician because it involves removing the control shaft. and the completed installation must be tested as to not interfere with the movement of the control shaft. Some aircraft were simpler that others to do. Some machining will need to be done depending on the aircraft model. Check out Radio Shack's Model: 275-1547 to give you an idea of what I used. In my case industrial grade products were used. Did a lot of Beechcaft, Pipers and Cessna. PTT was always a part of my custom installations. Having a machine shop was an integral part of my avionics shop. Closed shop in 1981. Have a Happy Thanksgiving.